Septic shock and the resultant multiple organ failure it induces is the leading cause of death in most intensive care units in the United States. Over 750,000 people become septic each year with a mortality rate of 30-40% and an approximate cost of $16.7 billion. Many studies in animal models and in patients who have died of sepsis have shown that sepsis induces extensive loss of white blood cells via apoptosis. This immune cell loss likely compromises the host's ability to fight infection and increases the risk of death from new infections. In addition, there are now multiple studies showing that uptake of apoptotic cells by macrophages and other phagocytic cells suppresses their ability to fight infection and contributes to immunosuppression. Furthermore, several studies in animal models of sepsis have shown that blocking apoptosis of lymphocytes improves overall survival. The Bcl-2 family of proteins have been shown to play a critical role in sepsis-induced lymphocyte apoptosis. Recently, over-expression of anti-apoptotic Bcl-2 family members (Bcl-2, Bcl-xL) or deletion of pro-apoptotic family members (Bim, Bid, and Puma) has been shown to decrease immune effector cell apoptosis and increase survival in animal models of sepsis. Preliminary work in our lab has shown that reduction of Bim expression using siRNA causes a dramatic decrease in sepsis-induced lymphocyte apoptosis and thus offers the potential to recapitulate the genetic deletion. The guiding hypothesis of the present proposal is that siRNA-mediated reduction of expression of pro-apoptotic Bcl-2 family members will prevent sepsis-induced apoptosis, ameliorate the sepsis-induced immune suppression, and improve survival. In addition, the molecular mechanisms and pathways of apoptotic cell death in sepsis will be examined using siRNA directed therapy. Finally, mice possessing a "humanized" immune system will be studied to determine the potential efficacy of siRNA therapy in a clinically relevant, translational model of sepsis.